Frequency stability improvement of a two-mode stabilized 633-nm He–Ne laser

The red side (lower-frequency) mode of a two-mode stabilized 633-nm He–Ne laser has been locked to the hyperfine structure of the P(33) line of 127I2 by means of frequency modulation spectroscopy enhanced by an external optical cavity. Both the red side and blue side (higher-frequency) modes of the laser exhibit a frequency stability of 2.3 × 10-11 τ-1/2. In addition, the frequency fluctuations of the blue side mode are detected by a Fabry–Perot cavity and compensated through an acousto-optic frequency shifter. The short-term stability of better than 3 × 10-11 is attained for integration times of between 2 × 10-3 and 2 × 10-1 s.